1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory, and is particularly used for a multi-level NAND-structured flash memory.
2. Description of the Related Art
A flash memory changes the amount of electric charges stored in a floating gate electrode of a memory cell to vary its threshold value, and stores data. For example, “1” corresponds to the threshold value of the memory cell being negative, and “0” corresponds to the threshold value being positive (e.g., refer to Jpn. Pat. Appln. KOKAI Publication No. 10-177797).
Recently, in order to decrease the value per bit or to increase a storage capacity, so-called multi-level flash memories have been developed to store multiple bit data in one memory cell. When 2-bit data is stored in one memory cell, four threshold distributions of the memory cell exist in accordance with the quaternary data.
Consequently, it is generally desired that the shape of the threshold distribution of the memory cell be narrow and sharp in view of a decrease in a power supply voltage and manufacturing variability. However, in order to obtain such a narrow and sharp threshold distribution, it is necessary to give specific consideration, for example, to narrow a step-up width of the write potential Vpgm, resulting in an adverse effect such as a lower write speed.
On the other hand, for example, a highly accurate and highly reliable multi-level flash memory can be manufactured in some manufacturing techniques. In such a case, it is not necessary to obtain the narrow and sharp threshold distribution at the cost of the write speed.
Furthermore, if attention is focused on the memory cells in one chip, only some memory cells are less reliable, while other cells are highly reliable in some cases. In such a case, it is not preferable in terms of yield ratio improvement and cost reduction that the chip be considered defective become of some less reliable memory cells.
Therefore, if the chip itself has a function whereby it can freely decide the shape of threshold distribution depending on characteristics of the memory cells, or a function to produce shapes of threshold distribution which are different in highly reliable memory cells and less reliable memory cells, products that have conventionally been treated as defective products can be non-defective products, which is effective for the yield ratio improvement and cost reduction.